This invention relates to a fuel control system for use in conjunction with a gas turbine engine, primarily a gas turbine engine constituting one of the engines of a twin or other multi-engine aircraft.
Considering a twin engine aircraft as the simplest example of a multi-engine aircraft, then in the event that an engine""s control system malfunctions in a way which results in over-speed of that engine then that engine produces excessive thrust by comparison with the thrust required in that operating condition and the aircraft can yaw. Such a situation arising during most of the flight cycle is unfortunate, but can normally be accommodated by shutting down the engine experiencing the excessive thrust. However, if one of the engines suffers an uncommanded increase in thrust during the pre-landing approach phase of the flight cycle then the yaw generated can prove more problematic.
If, as may well be the case, the yaw cannot be compensated for by the pilot by rudder control then the only option is to reduce the fuel supply to the engine which is experiencing the uncommanded increase in thrust and a rapid reduction in engine fuelling may well cause the engine to xe2x80x9cflameoutxe2x80x9d and thus to stop operating suddenly in a part of the flight cycle where there is no time to restart the engine. A sudden flameout of one engine will result in a thrust imbalance and probably a severe resultant yaw in the opposite direction. Such problems have been recognised in the past, and our co-pending patent application GB 2300451 discloses one way in which the flow of fuel to each engine can be controlled to avoid such difficulties. While the arrangement disclosed in our co-pending British patent application GB 2300451 admirably overcomes such operating difficulties, it does so by requiring a significant duplication of components in the engine fuel control system which adds to the weight, cost and complexity of the fuel system and the servicing requirements of the fuel system. It is an object of the present invention to provide a fuel control system for a gas turbine engine wherein the aforementioned disadvantages are mitigated in a simple and convenient manner.
In accordance with the present invention there is provided a fuel control system for a gas turbine engine comprising,
a main fuel metering valve for controlling the flow of fuel to a respective gas turbine engine in use, the valve including a valve control member moveable relative to a valve body by fluid pressure applied to a pressure chamber of the valve, to control the quantity of fuel supplied through the valve between zero in a valve closed condition and a maximum in a valve fully-open condition,
a first fluid control valve operable to control the application of fluid pressure to said pressure chamber of said metering valve to determine the setting of said metering valve and thus the quantity of fuel supplied to the gas turbine engine through the metering valve,
means for detecting an over-speed operating condition of the respective gas turbine engine in use, and,
a second fluid control valve moveable from a first condition to a second condition in response to detection of such over-speed condition, said second fluid control valve serving in said first condition to apply fluid pressure derived from said first fluid control valve to said pressure chamber of said metering valve, and serving in its said second condition to prevent the application of fluid pressure derived from said first fluid control valve to said pressure chamber of said metering valve and to vent said pressure chamber to permit said control member of said metering valve to move towards a valve closed position.
Preferably the passage through which fluid pressure in said pressure chamber of said metering valve is vented in said second condition of said second fluid control valve, includes a restrictor determining the rate at which said control member of said metering valve moves towards said metering valve closed condition.
Preferably said metering valve includes inlet port means connected to the supply of fluid pressure from the first fluid control valve other than through said second fluid control valve, and an internal connection between said inlet port means and said pressure chamber which opens, to supply fluid pressure from said first fluid control valve to said pressure chamber, in a position of said metering valve control member relative to the valve body in which there is a predetermined quantity of fuel supplied through the metering valve to said engine in use.